Just a Phase? Weakening Vertical Shear Instability Explains Class II Disk Morphologies: Simulations with dust coagulation, sedimentation, thermal relaxation, and backreaction

Abstract

The Vertical Shear Instability (VSI) is known to create turbulence and strong vertical mixing in protoplanetary disks if thermal relaxation is sufficiently fast. In simulations where this condition is met, VSI can loft dust particles to large altitudes, creating a vertically extended appearance in mock millimeter-wavelength observations, which is inconsistent with the morphology of the majority of observed class II disks. We present simulations of protoplanetary disks with VSI that are consistent with the observed thin-disk geometries, while maintaining the commonly observed bowl-shaped morphology in scattered light images at micrometer wavelength. We show that this outcome arises naturally when the effects of dust coagulation, sedimentation, and dust-gas thermal accommodation are taken into account. Sedimentation-driven coagulation removes large amounts of dust from the disk atmosphere, in the process slowing down the dust-driven cooling of the gas. At the same time, a dense midplane layer of millimeter-sized grains forms, which exerts aerodynamic drag on the gas. This results in the termination of the VSI's corrugation mode. Only weak VSI activity remains in the upper and lower hemispheres. These processes occur on the typical dust growth timescale and suppress strong VSI-induced turbulence within a few hundred thousand years. VSI could thus generally be restricted to the class I evolutionary stages of protoplanetary disks.